Citrus, or any, fruit cracking is caused by an uneven or irregular watering, such as when you have a drought and then suddenly you get an abundance of rain. This moisture causes the skin to swell and eventually crack or split open. This article for cracking oranges (which is similar in cause, effect, and treatment) should help: https://www.gardeningknowhow.com/edible/fruits/oranges/oranges-splitting.htm